1. Field of the Invention
The present invention relates to a motor-driven injection molding machine using a drive motor (servomotor) at a drive section.
2. Description of the Relevant Art
Generally, a motor-driven injection molding machine using a servomotor at a drive section potentially involves malfunction or breakage of the servomotor due to overheat arising under certain molding conditions or under a certain environment, since the servomotor generates heat according to load current. Therefore, the motor-driven injection molding machine usually employs measures for protection of the servomotor. Specifically, a heat generation value of the servomotor is detected. When the detected heat generation value reaches a preset stopping point, the value is interpreted as indicating overload, and the servomotor is forcibly stopped to thereby protect the servomotor.
For an injection molding machine, stoppage of a servomotor during molding means interruption of a molding process, which may have an adverse effect on the molding process once resumed; e.g., lowering the operating ratio. Therefore, interruption of a molding process must be avoided to the extent possible. In order to meet the end, for example, Japanese Patent Application Laid-Open (kokai) No. 11(1999)-235743 proposes a method for checking preset operating conditions. According to the proposed method, the injection molding machine is operated on a trial basis in order to judge, from load current flowing through a servomotor, whether or not the servomotor operates in an overloaded unstable region. When the servomotor operates in the overloaded unstable region, the preset operating conditions are reviewed so as to impart appropriate allowance thereto, to thereby avoid interruption of operation (molding process) during actual molding.
The above-mentioned conventional method involves the following problems.
First, since operating conditions are reviewed merely at the stage of trial operation, the review fails to reflect operating conditions during actual molding. Thus, unnecessary interruption of molding may occur, so that satisfactory stability and reliability cannot be attained.
Second, since the preset operating conditions are reviewed at the stage of trial operation so as to impart appropriate allowance thereto, the conventional method may fail to set optimum operating conditions, resulting in impaired molding quality or productivity.
An object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of reliably avoiding unnecessary interruption of molding, to thereby improve stability and reliability.
Another object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of always setting and maintaining optimum molding conditions to thereby avoid impairment in molding quality and productivity.
To achieve the above objects, the present invention provides a method for controlling a motor-driven injection molding machine which includes a drive motor provided at a drive section, the method comprising: obtaining a load level of the drive section for each molding cycle; determining whether the load level has entered an overload region; and performing, when the load level has entered the overload region, cycle-time prolongation processing in order to obtain a cycle prolongation time corresponding to the load level and to prolong a period of the molding cycle by the cycle prolongation time.